Q: Can I use milky spore to prevent recurring problems with Japanese beetles in my lawn? --Beth Thomas, Morton Grove A: Milky spore disease can reduce the number of Japanese beetle larvae feeding on your turf grass. But adding the bacterial spores to your soil is a process, not a one-time treatment. Milky spore disease affects only Japanese beetle grubs, but not white grubs, which also are common in Illinois lawns. For it to work, you must have a large number of only Japanese beetle grubs. For best effect, the soil temperature when you apply must be just right, between 60 and 70 degrees. While some grubs can be killed following the first application, in our area, it usually takes up to five years for the bacteria to spread and reach levels high enough to provide control, and the progress is affected by soil temperature each year. In the meantime, you can't apply any other grub-killing insecticide. The process takes time because the Japanese beetle grubs have to ingest the bacteria for the number of spores to increase. When the number of spores in each grub becomes high enough, the grub dies and its decomposing body releases the additional spores into the soil. At the same time, infected grubs moving through the soil spread the spores. With too few grubs, the number of spores remains about the same. The more grubs you have, the faster the process works. But there are potential problems. The first is uncertainty. If after several years you have too few grubs to cause real damage to your lawn, you can't be sure your investment in milky spore disease caused their decline. The real reasons your problem is now under control could be long periods of subzero temperatures, summer droughts or a variety of predators, such as skunks and raccoons. Another potential problem is the long period the milky spore bacteria persist in the soil. Research suggests it can be as long as 20 years. Some experts are concerned this persistence might allow Japanese beetle larvae to evolve resistance to the disease. Grubs are best treated only when the problem is large enough. If you had enough grubs of any kind (10 to 12 per square foot) and damage (sod pulls back like carpet) to justify using a chemical control, you might consider using only one product that effectively controls both kinds of grubs. One of the best ways to make your lawn less hospitable to grubs is to avoid overwatering. Grubs thrive in damp soil. For the current recommendations on the best ways to control grubs in turf grass, call the Plant Information Service at 847-835-0972.
Robin Carlson, Chicago Botanic Garden